Pulsed lasers are used in nonlinear bio-imaging techniques such as multi-photon fluorescence excitation microscopy (MPFM) or two-photon fluorescence excitation microscopy (2PFM), which is a type of MPFM. In 2PFM, two photons of the same or different energy are absorbed by a molecule and the fluorescence from the molecule upon relaxation is collected with a highly sensitive detector such as a photomultiplier tube. Because the probability of near simultaneous absorption of two photons is low, a high flux of excitation photons is usually needed, and therefore an ultrafast (for example, femtosecond) pulsed laser is typically used. An example of a suitable pulsed light source is a Ti:sapphire laser, which can operate from 650-1100 nm with pulse widths of about 100-150 fs, repetition frequencies of about 70-80 MHz, and at several Watts of power (for example, 3 W).